Piezoelectric transducer assembly with phase shifting accoustical parts

ABSTRACT

A piezoelectric element which resonates at a predetermined ultrasonic frequency is mounted in a housing structure formed by a cup and insert structure. A resonant cavity is defined in the cup and the element is held adjacent thereto by bosses formed on a pair of mounting chips. The bosses contact the element along its node so that it may vibrate in an unrestrained manner and the mounting chips define acoustical ports therewith on each side of the element which function to delay and thereby phase shift acoustical waves as they pass therethrough so that acoustical waves at the element&#39;&#39;s resonant frequency combine in phase at the element&#39;&#39;s surface. The mounting chips and bosses also function as electrodes and electrical leads are electrically connected thereto for connecting the transducer assembly formed to external circuitry.

United States Patent Schweitzer et a1.

PIEZOELECTRIC TRANSDUCER ASSEMBLY WITH PHASE SHIFIING ACCOUSTICAL PARTSInventors: John C. Schweitzer; Edward M. Ju-

nak, both of Grand Junction, Colo.

Della Products, Inc., Grand Junction, Colo.

Filed: March 2, 1971 Appl. No.: 120,252

Assignee:

U.S. Cl. ..3l0/8.2, 310/85, 310/91 Int. Cl. .......H01v 7/00 Field ofSearch ..310/8.5, 8.6, 8.8, 9.1, 9.2,

310/95, 8.2; 340/10; 179/1 10 A, 110 D, llOF References Cited UNITEDSTATES PATENTS [15] 3,704,385 14 1 Nov. 28, 1972 7/1967 Dundon et a1...3 l0/9.1 3/1964 Kritz ..3 [0/91 X 1 1 ABSTRACT A piezoelectric elementwhich resonates at a predetermined ultrasonic frequency is mounted in ahousing structure formed by a cup and insert structure. A resonantcavity is defined in the cup and the element is held adjacent thereto bybosses formed on a pair of mounting chips. The bosses contact theelement along its node so that it may vibrate in an unrestrained mannerand the mounting chips define acoustical ports therewith on each side ofthe element which function to delay and thereby phase shift acousticalwaves as they pass therethrough so that acoustical waves at the elementsresonant frequency combine in phase at the elements surface. Themounting chips and bosses also function as electrodes and electricalleads areelectrically connected thereto for connecting the transducerassembly formed to external circuitry.

13 Claims, 7 Drawing Figures PATENTED NOV 28 I972 FIG. 5

INVENTORS JOHN C. SCHWEITZER E QWARD M. JUNAK flaw 7 ATTORNEYS FIG. 6

PIEZOELECTRIC TRANSDUCER ASSEMBLY WITH PHASE SHIFTING ACCOUSTICAL PARTSThe present invention relates to transducers and more particularly to animproved piezoelectric transducer assembly which is highly sensitive andmay be employed as an ultrasonic transducer for converting electricalsignals to high frequency inaudible sound waves or vice versa.

Piezoelectricity is pressure electricity and piezoelectric behavior isthe characteristic of materials to deform upon the application ofelectrical signals or conversely to develop electricity wheneverdeformed by the application of pressure. Certain natural occurringcrystals are known to exhibit piezoelectric behavior. Also, it is knownthat piezoelectric behavior may be induced in ceramic materials by useof a polarizing treatment. Piezoelectric materials whether naturallyoccurring or man created are anisotropic, i.e., the magnitude of thepiezoelectric properties they exhibit vary with direction in thematerials. For example, a ceramic crystal may be polarized in a givendirection so that application of an electrical field of one polarityalong this direction causes expansion and application of a field'ofopposite polarity causes contraction while application of an electricalfield perpendicularly to the direction of polarization has substantiallyno effect on the piezoelectric material.

Heretofore, piezoelectric materials have been utilized in ultrasonictransducer assemblies. One type of piezoelectric element which has beenemployed in ultrasonic applications is a flexing type of piezoelectricmaterial which bends along a given plane or dimension about a node inresponse to an electric field applied perpendicularly to the materialsgiven plane or dimension. An inherent disadvantage with the use of sucha piezoelectric element in ultrasonic applications is that as theportion of the element on one side of the node vibrates in one directionthe portion of the element on the other side of the mode always vibratesin an opposite direction. Thus, as a compression wave is being createdon one side of the node a rarefaction wave is always being created onthe other side of the node. As a result, the compression and rarefactionwaves created on opposite sides of the elements node tend todestructively interfere with each other and cancel out the acousticaloutput of such piezoelectric elements. Thus, the response andsensitivity of such a piezoelectric element is significantly decreasedby this destructive interference effect and its effectiveness inultrasonic applications whether for converting acoustical to electricalenergy or vice versa is severely limited.

It is, accordingly, an object of the present invention to provide animproved piezoelectric transducer assembly which is constructed so as tonot be subject to destructive interference whereby to obviate theaforementioned disadvantage inherent with prior art piezoelectrictransducer assemblies.

It is, further, an object of the present invention to provide animproved piezoelectric ultrasonic transducer which is characterized byhaving a relatively high sensitivity and response both for convertingacoustical energy to electrical energy and vice versa.

It is, also, an object of the present invention to pro vide an improvedpiezoelectric ultrasonic transducer assembly which employs a type ofpiezoelectric element which bends or flexes about a node and which ischaracterized by being constructed so as to not be subject todestructive acoustical interference whereby to have relatively highsensitivity and response charac teristics.

It is, additionally, an object of the present invention to provide animproved piezoelectric ultrasonic transducer assembly which employs atype of piezoelectric element which bends or flexes about a node andwhich is constructed to phase shifi acoustical waves generated ondifferent sides of the node so as to eliminate destructive acousticalinterference.

Additional objects of the present invention reside in the specificconstruction of the exemplary transducer assembly hereinafterparticularly described in the specification and shown in the severaldrawings.

in accomplishing these and other objects, there is provided inaccordance with the present invention a transducer assembly formed by apiezoelectric element having a predetermined ultrasonic resonantfrequency at which it vibrates by bending along its sides. The elementhas a node which is substantially free from vibratory motion so that theelement portion on one side of the node always vibrates in a directionopposite to the direction of vibration of the element portion on theother side of the node. Structure is provided which mounts the elementfor substantially unrestricted vibration and electrodes are positionedto sense an electrical signal on the element or apply an electricalsignal thereto. Electrical leads are connected to the electrodes forconnecting the transducer assembly to external electricalcircuitry and aresonant cavity is defined in the transducer assembly which amplifiesvibrations at the resonant frequency of the piezoelectric element.Acoustical parts are formed adjacent each side of the piezoelectricelement to phase shift acoustical waves at the resonant frequencytraveling from one side of the node to the other so that acousticalwaves at the resonant frequency combine in phase on each side of theelement instead of destructively interfering with each other. Thus,there is provided an improved piezoelectric transducer assembly which isconstructed to eliminate destructive acoustical interference so as tohave relatively high sensitivity and response characteristics both forconverting acoustical to electrical energy and vice versa.

A better understanding of the present invention may be had from thefollowing detailed description when read in conjunction with theaccompanying drawings, in which:

FIG. 1 is a top plan view of a transducer assembly according to thepresent invention;

FIG. 2 is a bottom view of the transducer assembly of FIG. 1;

FIG. 3 is a side view of the transducer assembly of FIG. 1; w v FIG. 4is a sectional view taken along the line 4-4 of FIG. 1;

FIG. 5 is a sectional view taken along the line 5-5 of FIG. 1;

FIG. 6 is a top plan view of the piezoelectric element of FIG. 1 withthe position of its node indicated thereon in a dashed line andillustrating amounting chip positionedto contact the piezoelectricelement along its node; and FIG. 7 is a sectional view taken along theline 77 of FIG. 6.

Referring to the drawings in more detail, there is shown a transducerassembly generally designated by the numeral made up of an outer cupportion 11, a cup insert 12, a support ring 13, a pair of mounting chips14 and a piezoelectric element 15. The piezoelectric element 15 is inthe form of a thin flat layer arrangement and is preferably rectangularin shape. The piezoelectric element is of the so called bender typewhich in response to an electrical field applied perpendicularly to itsflat surfaces it flexes or bends as shown in FIG. 7 about its node. Theelement 15 may be made from any suitable crystal, ceramic or otherpiezoelectric material and can be a BIMORPH" bender type ofpiezoelectric element made by the Clevite Corporation. BIMORPH is atrade name of the Clevite Corporation and such BIMORPH" bender typepiezoelectric elements are generally made by securing together twotransverse-expander plates in a sandwich type layer construction.

The piezoelectric element 15 is designed to resonate at a frequency inthe ultrasonic range and have a node located on its surface at apredetermined and known location between its center and its outerperipheral edge. As shown in FIG. 6, the node is indicated by the dashedcircular line 20. It is noted that the node on a vibrating element, suchas vibrating piezoelectric element, is the point, line or surface on theelement which is always free or relatively free of vibratory motion, andthat the node line divides each side of the piezoelectric element 15into a center area within the node line 20 and an edge area outside ofthe line 20. FIG. 7 illustrates the vibratory motion of the element 15in two different positions, with one position being shown in dashedlines while the other position is shown in solid lines.

The piezoelectric element 15 is mounted in the transducer assembly 10 bymeans of the mounting chips 14. The mounting chips 14 are made of anelectrical conductive material, such as a suitable metal, and also serveas electrode plates for picking off or applying electrical signals tothe piezoelectric element 15. Pairs of similarly shaped bosses 21 madeof electrically conductive material are formed on one side of each ofthe mounting chips 14 for contacting the element 15. The

bosses 21 are preferably spaced apart a distance equal to the diameterof the circle defined by the node line 20 and are shaped and positionedfor making point contacts with the element 15 exactly along the nodeline 20. The bosses 21 serve dual purposes first, providing electricalcontact points for picking off or applying electrical signals to thepiezoelectric element 15 and secondly, holding the mounting chips 14 andthe element 15 in a predetermined spaced apart layer relationship so asto define air vents or acoustical ports 22 along each side of thepiezoelectric element 15. It is noted that by mounting the piezoelectricelement 15 between the mounting chips 14 by means of bosses 21 formed onthe chips 14 which contact the node of the element 15, the element 15 isheld so that it is free to vibrate in a substantially unrestrictedmanner since the bosses 21 only contact the element 15 at points whichare substantially free of vibratory motion.

As mentioned, acoustical or air ports 22 are formed along each side ofthe piezoelectric element 15 by the spaced apart mounting arrangement ofthe element 15 between the mounting chips 14. The effective size ofthese air ports 22 is determined by the shaping and dimensioning of theelement 15, the chips 14 and the bosses 21. The function of the airports 22 is to physically restrict and thereby delay to a predeterminedextent acoustical waves generated by a portion of the piezoelectricelement 17 so that the acoustical waves generated by the portion of theelement 17 within the node defined by dashed line 20 reinforce theacoustical waves generated by the edges of the element 17 instead ofdestructively interfering therewith. As before mentioned, the reasonsuch a phase shift is necessary is that when the portion of thepiezoelectric element 17 within the nodal line 20 is moving in onedirection its edges are moving in the opposite direction so that while acompression wave is being generated by the center portion of the element15 a rarefaction wave is being generated by its edges and vice versa.These simultaneously generated compression and rarefaction waves on thesame side of the element 15 will destructively interfere and causecancellation of the acoustic output unless they are phase shiftedrelative to each other so as to combine in phase.

In the exemplary transducer herein described, the mounting chips 14 aredesigned to be substantially flat and are positioned adjacent the centerportion of the element 15 in a spaced apart parallel relationship to theat rest position of the piezoelectric element 15. Thereby, the air ports22 are designed to physically restrict and delay the movement ofacoustical waves out from and into the center of the element 15 so thatthe acoustical waves generated or sensed by the edges of the element 15reinforce and add with the acoustical waves generated or sensed by thecenter of the element 15. For example, as a compression wave generatedby one side of the center portion of the element 15 travels outward tothe edges of the element 15, the compression wave is time delayed byone-half wave length by the physical restriction provided by the airports 22 so that by the time the compression wave arrives at the elementedges the edges on the same side of the element 15 are also moving in adirection which generates a compression wave. Thus, the acoustical wavesgenerated by the center portion of the piezoelectric element 15 and itsedges are in phase and consequently combine to produce a reinforced,strong acoustical output signal.

The appropriate sizing and shaping for the air or acoustical ports 22depends on the size of the piezoelectric element 15 and its naturalresonant frequency. In an exemplary transducer assembly constructed, thepiezoelectric element 15 was square shaped having a side dimension of0.317 inches, a width of 0.024 inches, and a resonant frequency of 36kilo herz (KHz). To form appropriate air ports 22 which were operable todelay and hence phase shift a 36 KHz acoustic wave one-half wave lengthas it passed therethrough, the mounting chips 14 were made to be flatsubstantially square shaped metallic members having side dimensions of0.2 inches, width of 0.015 inches, and having rounded to coin cornerswith a diagonal dimension of 0.232 inches between opposite comers. Thepointed bosses 21 formed on the chips 14 had a height of 0.008 inches, awidth of 0.031 inches and were spaced apart to contact the element 15 atits node on opposite sides of a diameter drawn through the center of theelement 15. The element 15 employed was designed to have a circular nodehaving an 0.216 inch diameter and thus the spacing of the bosses 21 were0.216 inches apart. The point contacts of the pairs of bosses 21 formedon each mounting chip 14 further were positioned along a diagonal drawnthrough opposite comers of the chip 14 and were positioned 0.008 inchesin from the comer edge of the chip 14. It is pointed out that variousother structural arrangements having other dimensions may be employed todefine appropriate acoustical ports 22 and that such other arrangementsare merely equivalent arrangements of the detailed arrangement heredescribed. The detailed description above given is provided for thepurpose of illustrating one suitable arrangement for use with apiezoelectric element having a 36 KHz resonant frequency. It is notedthat a piezoelectric element having a resonant frequency other than 36KHz may be employed and that in such a case the dimensioning of thechips 14 and bosses 21 would be changed to provide suitable ports 22 foruse with the specific piezoelectric element employed.

As before mentioned, the piezoelectric element 15 is mounted in thetransducer assembly by means of the mounting chips 14. This isaccomplished by including cross members 30 in the support ring structure13 which intersect preferably at right angles to each other in thecenter of the ring 13. The cross members 30 are notched at their centerto define a square shaped recessed portion 31 slightly larger than theelement so as to provide clearance for the vibrating element 15. Thecross members also form a centrally located seat means 32 for onemounting chip 14 so that the chips 14 and element 15 may be mounted inthe ring 13 by the following procedure: seating one chip 14 centrally inthe seat 32 with the bosses 21 formed thereon extending outward,positioning the element 15 on these bosses 21 with the bosses 21contacting the element node line 20, and then positioning the other chip14 adjacent the element 15 with its bosses 21 contacting the element 15along its node line 20. Thereby, the piezoelectric element 15 issupported in the ring 13 in such a way that it is free to vibrate in anunrestrained manner since its only contacts with mounting structure arethe contacts made by the bosses 21 along the elements node. It is notedthat the mounting chips 14 are preferably mounted relative to each otherso that imaginary lines drawn through the bosses 21 on each of the chip14 are perpendicular to each other. Thereby, the element 15 is supportedbetween the chips 14 in a saddle type mounting construction as shown.

The cup 11 of the transducer assembly 10 forms the housing portion ofthe assembly and has an outward extending rim formed on its open end. Ahole 40 is formed centrally in the other end of the cup for receiving anannular projecting portion 41 of the cup insert 12. The cup insert 12 isfitted in the bottom of the cup 11 with its annular portion 41 extendingthrough the hole 40. The cup insert 12 is shaped to define a resonantcavity 42 which is appropriately sized to correspond with the resonantfrequency of the piezoelectric element 15. With the cup insert 12 inposition, the support ring 13 with the chips 14 and the element 15correctly positioned thereon is fitted in the cup 1 1 with its annularring portion positioned against the cup insert 12. Adhesive or cementmay then be applied to secure the cup insert 12 and support ring 13 inplace.

To hold the mounting chips 14 and the element 15 in place in theirstacked relationship, a straight piece of rigid wire 43, such as springor music wire, is passed through matching holes 44 formed in the cup 11and secured at point 45, such as by soldering, to the outer mountingchip 14 as shown. At least one of the ends of the wire 43 is thencemented or otherwise secured to the side of the cup 11 at the point itpasses through a hole 44.

To complete the assembly of the transducer assembly 10, electrical leads50 and 51 are electrically connected to the electrodes provided by theelectrically conductive mounting chips 14. The lead 50 is connected tothe inner chip 14 by passing the lead 50 through holes 52 and 53 formed,respectively, in the cup insert projection 41 and the cross members 30,and soldering or otherwise electrically connecting the lead 50 to theinner chip 14. The lead 51 is soldered or otherwise electricallyconnected directly to the cup 1 1. An electrical connection is, thus,made to the outer chip 14 through the cup 1 1 which is made of anelectrically conductive material, such as metal, and the electricallyconductive wire 43 which connects both the cup 11 and the outer mountingchip 14. It is noted that in order to electrically isolate the lowerchip electrode 14 from the cup 11, and thus from the upper electrodechip 14, the cup insert 12 and the support ring 13 are made of anelectrical insulating material or are otherwise electrically isolatedfrom the cup 1 1. Additionally, an adhesive may be applied across theinside opening of the hole 52 in the cup insert projection 41, first tosecure the lead 50 in place so that it does not move the lower mountingchip 14 and secondly, to seal this hole 52 in the resonant cavity 42.

In use and operation of the transducer assembly 10, the cup 13 ispreferably mounted in a shock absorbent material, such as foam rubber,so as to be unaffected by extraneous vibrations. in use as an acousticto electric energy transducer, the leads 50 and 51 are connected to anelectrical measuring instrument, such as an extremely accuratevoltmeter. The transducer assembly 10 then operates to sense acousticvibrations at the resonant frequency for which it is designed. Forexample, if the transducer assembly 10 is designed to sense 36 KHzacoustic waves, the piezoelectric element 15 vibrates at 36 KHz uponsensing a 36 KHZ acoustical wave and this vibration causes a voltagesignal to appear across the width of the element 15 which is picked offby the leads 50 and 51 through the electrical contacts provided by thebosses 21. It is noted that since the phase shift means provided by theair ports 22 appropriately phase shifts the 36 KHz acoustical signalbeing sensed so that no destructive interference of the signal occurs atthe vibrating surface of the element 15 that the transducer assembly 10is highly sensitive and produces a readily measurable electrical outputwhich is transmitted by the leads 50 and 51 to the electrical measuringdevice.

in the case where the transducer assembly 10 is employed as an electricto acoustic energy transducer, the leads 50 and 51 are connected to anelectrical device which produces or generates an electrical output atapproximately the resonant frequency of the piezoelectric element 15. Ifthe element 15 has a resonant frequency of 36 [(112, 36 KHz electricalsignals applied across its width cause the element 15 to vibrate at itsresonant frequency. Thereby, the element generates an acoustical outputat its resonant frequency. Since the air ports 22 are designed to phaseshift by one-half wave length the acoustical waves generated by thecenter portion of the element 15 with respect to the acoustical wavesgenerated by its outer edge, the acoustical waves generated reinforceeach other, instead of destructively interfering, to produce a strongacoustical output.

Thus, there is provided an improved piezoelectric device suitable foruse as an ultrasonic transducer which is constructed to eliminatedestructive acoustical interference so as to have relatively highsensitivity and response characteristics both for converting acousticalenergy to electrical energy and vice versa. The destructive interferenceis eliminated by providing means for physically delaying acousticalwaves generated or received so that they combine in phase at the surfaceof the piezoelectric element to reinforce the acoustical signal. It isto be understood that in accordance with the spirit of the presentinvention various changes in shaping and dimensioning may be made inconstructing the means which provide the acoustical phase shift ordelay.

What is claimed is:

l. A transducer assembly, comprising:

a piezoelectric element having a predetermined resonant frequency atwhich it vibrates, said element being substantially flat and having anode which is substantially free of vibratory motion;

structure means mounting said element for substantially unrestrictedvibration, said structure means including a pair of flat mounting chipspositioned on opposite sides of said flat element and havingelectrically conductive contact points formed thereon which contact saidelement along its node and hold said mounting chips and said element ina mutually parallel spaced apart relationship, said contact pointsproviding electrode means to pick up an electrical signal generated byvibration of said element or to apply an electrical signal to saidelement to cause vibration thereof; and

acoustical port means formed adjacent said element for delaying andthereby shifting into phase acoustical waves generated and sensed bysaid element which are at said predetermined resonant frequency so thatacoustical waves at said predetennined resonant frequency combine inphase at the surface of said element, said acoustical port means beingformed adjacent said element by the surfaces of said spaced apartmounting chips and element and being sized as a function of saidpredetermined resonant frequency.

2. The invention recited in claim 1, wherein each of said mounting chipsis electrically conductive and including electrical leads connected tosaid mounting chips for connecting said transducer assembly to externalelectrical circuitry.

3. The invention recited in claim I, wherein the node of said elementwhich is substantially free of vibratory motion divides said elementinto two portions and the portion of said element to one side of saidnode always vibrates in a direction opposite to the direction ofvibration of the portion of said element to the other side of said node,and said acoustical port means are operable to phase shift an acousticalwave at said predetermined resonant frequency as it travels from oneside of said node to the other by one-half wave length so thatacoustical waves generated or sensed at said resonant frequencyreinforce each other.

4. The invention recited in claim 1, wherein said element vibrates at aresonant frequency in the ultrasonic range.

5. The invention recited in claim 4, wherein said element is a bendertype piezoelectric element.

6. The invention recited in claim 4, wherein said element is a bendertype piezoelectric element fafifia'a's two transverse expander platessecured together.

7. An ultrasonic transducer assembly, comprisin g:

a piezoelectric element having a predetermined ultrasonic resonantfrequency at which it vibrates, said element being substantially flatand having a node which is substantially free of vibratory motion;

first structure means defining a resonant cavity which is designed toamplify vibrations at said resonant frequency;

second structure means counting said element for substantiallyunrestricted vibration adjacent said resonant cavity, said secondstructure means including a pair of electrically conductive flatmounting chips positioned on opposite sides of said flat element andhaving electrically conductive contact points formed thereon whichcontact said element along its node and hold said mounting chips andsaid element in a mutually parallel spaced apart relationship, saidcontact points providing electrode means to pick up an electrical signalgenerated on said element or apply an electrical signal thereto;

electrical leads connected to said mounting chips for connecting saidtransducer assembly to external electrical circuitry; and

acoustical port means formed adjacent said element for delaying andthereby shifting into phase acoustical waves generated and sensed bysaid element which are at said predetermined resonant frequency so thatacoustical waves at said predetennined resonant frequency combine inphase at the surface of said element, said acoustical port means beingformed adjacent said element by the surfaces of said spaced apartmounting chips and element.

8. The invention recited in claim 7, wherein the node of said elementwhich is substantially free of vibratory motion divides said elementinto two portions and the portion of said element on one side of saidnode always vibrates in a direction opposite to the direction ofvibration of the portion of said element on the other side of said node,and said acoustical port means are operable to phase shift an acousticalwave at said predetermined resonant frequency as it travels from oneside of said node to the other by one-half wave length so thatacoustical waves generated or sensed at said resonant frequencyreinforce each other.

9. The invention recited in claim 8, wherein said element is a bendertype piezoelectric element formed by two transverse expander platessecured together.

10. An ultrasonic transducer assembly, comprising:

a bender type piezoelectric element having two sides, said elementhaving a predetermined ultrasonic resonant frequency at which itvibrates by bending along its sides, said element having a node which issubstantially free from vibratory motion and which divides each side ofsaid element into a center area within said node and an edge areaoutside of said node so that said center element area always vibrates ina direction opposite to the direction of vibration of said edge elementarea;

structure means mounting said element for substantially unrestrictedvibration, said structure means including electrode means positioned topick up an electrical signal on said element or apply an electricalsignal thereto;

electrical leads connected to said electrode means for connecting saidtransducer assembly to external electrical circuitry; and,

acoustical port means adjacent each side of said element for delayingand thereby phase shifting acoustical waves generated and sensed by saidelement, each of said acoustical port means being operable to phaseshift an acoustical wave at said predetermined resonant frequency as ittravels between said center and edge element areas onehalf wave lengthso that acoustical waves at said predetermined resonant frequencycombine in phase on each side of said element.

11. the invention recited in claim 10, including second structure meansdefining a resonant cavity, said resonant cavity being formed adjacentsaid element and being designed to amplify vibrations at said resonantfrequency.

12. The invention recited in claim 10, wherein said structure meanscomprises a pair of flat mounting chips positioned on opposite sides ofsaid element and having electrically conductive contact points thereonwhich contact said element along its node and hold said mounting chipsand said element in a mutually parallel spaced apart relationship, saidcontact means providing said electrode means, said acoustical port meansbeing formed adjacent each side of said element by the surfaces of saidspaced apart mounting chips and element.

13. An ultrasonic transducer assembly, comprising:

a piezoelectric element having a predetermined resonant frequency atwhich it vibrates, said element having a node substantially free fromvibratory motion which divides said element into two portions and theportion of said element to one side of said node always vibrating in adirection opposite to the direction of vibration of the portion of saidelement to the other side of said node;

structure means mounting said element for substantially unrestrictedvibration, said structure means including electrode means positioned topick up an electrical signal on said element or apply an electricalsignal thereto; and

acoustical port means adjacent said element for delaying and therebyphase shifting acoustical waves generated and sensed by said element,said acoustical port means being operable to phase shift an acousticalwave at said predetermined resonant frequency as it travels from oneside of said node to the other by one-half wave length so thatacoustical waves at said predetermined resonant frequency goll l blllein phgse at said element.

1. A transducer assembly, comprising: a piezoelectric element having apredetermined resonant frequency at which it vibrates, said elementbeing substantially flat and having a node which is substantially freeof vibratory motion; structure means mounting said element forsubstantially unrestricted vibration, said structure means including apair of flat mounting chips positioned on opposite sides of said flatelement and having electrically conductive contact points formed thereonwhich contact said element along its node and hold said mounting chipsand said element in a mutually parallel spaCed apart relationship, saidcontact points providing electrode means to pick up an electrical signalgenerated by vibration of said element or to apply an electrical signalto said element to cause vibration thereof; and acoustical port meansformed adjacent said element for delaying and thereby shifting intophase acoustical waves generated and sensed by said element which are atsaid predetermined resonant frequency so that acoustical waves at saidpredetermined resonant frequency combine in phase at the surface of saidelement, said acoustical port means being formed adjacent said elementby the surfaces of said spaced apart mounting chips and element andbeing sized as a function of said predetermined resonant frequency. 2.The invention recited in claim 1, wherein each of said mounting chips iselectrically conductive and including electrical leads connected to saidmounting chips for connecting said transducer assembly to externalelectrical circuitry.
 3. The invention recited in claim 1, wherein thenode of said element which is substantially free of vibratory motiondivides said element into two portions and the portion of said elementto one side of said node always vibrates in a direction opposite to thedirection of vibration of the portion of said element to the other sideof said node, and said acoustical port means are operable to phase shiftan acoustical wave at said predetermined resonant frequency as ittravels from one side of said node to the other by one-half wave lengthso that acoustical waves generated or sensed at said resonant frequencyreinforce each other.
 4. The invention recited in claim 1, wherein saidelement vibrates at a resonant frequency in the ultrasonic range.
 5. Theinvention recited in claim 4, wherein said element is a bender typepiezoelectric element.
 7. An ultrasonic transducer assembly, comprising:a piezoelectric element having a predetermined ultrasonic resonantfrequency at which it vibrates, said element being substantially flatand having a node which is substantially free of vibratory motion; firststructure means defining a resonant cavity which is designed to amplifyvibrations at said resonant frequency; second structure means countingsaid element for substantially unrestricted vibration adjacent saidresonant cavity, said second structure means including a pair ofelectrically conductive flat mounting chips positioned on opposite sidesof said flat element and having electrically conductive contact pointsformed thereon which contact said element along its node and hold saidmounting chips and said element in a mutually parallel spaced apartrelationship, said contact points providing electrode means to pick upan electrical signal generated on said element or apply an electricalsignal thereto; electrical leads connected to said mounting chips forconnecting said transducer assembly to external electrical circuitry;and acoustical port means formed adjacent said element for delaying andthereby shifting into phase acoustical waves generated and sensed bysaid element which are at said predetermined resonant frequency so thatacoustical waves at said predetermined resonant frequency combine inphase at the surface of said element, said acoustical port means beingformed adjacent said element by the surfaces of said spaced apartmounting chips and element.
 7. The invention recited in claim 4, whereinsaid element is a bender type piezoelectric element formed by twotransverse expander plates secured together.
 8. The invention recited inclaim 7, wherein the node of said element which is substantially free ofvibratory motion divides said element into two portions and the portionof said element on one side of said node always vibrates in a directionopposite to the direction of vibration of the portion of said element onthe other side of said node, and said acoustical port means are operableto phase shift an acoustical wave at said predetermined resonantfrequency as It travels from one side of said node to the other byone-half wave length so that acoustical waves generated or sensed atsaid resonant frequency reinforce each other.
 9. The invention recitedin claim 8, wherein said element is a bender type piezoelectric elementformed by two transverse expander plates secured together.
 10. Anultrasonic transducer assembly, comprising: a bender type piezoelectricelement having two sides, said element having a predetermined ultrasonicresonant frequency at which it vibrates by bending along its sides, saidelement having a node which is substantially free from vibratory motionand which divides each side of said element into a center area withinsaid node and an edge area outside of said node so that said centerelement area always vibrates in a direction opposite to the direction ofvibration of said edge element area; structure means mounting saidelement for substantially unrestricted vibration, said structure meansincluding electrode means positioned to pick up an electrical signal onsaid element or apply an electrical signal thereto; electrical leadsconnected to said electrode means for connecting said transducerassembly to external electrical circuitry; and, acoustical port meansadjacent each side of said element for delaying and thereby phaseshifting acoustical waves generated and sensed by said element, each ofsaid acoustical port means being operable to phase shift an acousticalwave at said predetermined resonant frequency as it travels between saidcenter and edge element areas one-half wave length so that acousticalwaves at said predetermined resonant frequency combine in phase on eachside of said element.
 11. the invention recited in claim 10, includingsecond structure means defining a resonant cavity, said resonant cavitybeing formed adjacent said element and being designed to amplifyvibrations at said resonant frequency.
 12. The invention recited inclaim 10, wherein said structure means comprises a pair of flat mountingchips positioned on opposite sides of said element and havingelectrically conductive contact points thereon which contact saidelement along its node and hold said mounting chips and said element ina mutually parallel spaced apart relationship, said contact meansproviding said electrode means, said acoustical port means being formedadjacent each side of said element by the surfaces of said spaced apartmounting chips and element.
 13. An ultrasonic transducer assembly,comprising: a piezoelectric element having a predetermined resonantfrequency at which it vibrates, said element having a node substantiallyfree from vibratory motion which divides said element into two portionsand the portion of said element to one side of said node alwaysvibrating in a direction opposite to the direction of vibration of theportion of said element to the other side of said node; structure meansmounting said element for substantially unrestricted vibration, saidstructure means including electrode means positioned to pick up anelectrical signal on said element or apply an electrical signal thereto;and acoustical port means adjacent said element for delaying and therebyphase shifting acoustical waves generated and sensed by said element,said acoustical port means being operable to phase shift an acousticalwave at said predetermined resonant frequency as it travels from oneside of said node to the other by one-half wave length so thatacoustical waves at said predetermined resonant frequency combine inphase at said element.